Post-Variscan evolution of the lithosphere in the Rhine Graben area: constraints from subsidence modelling

Abstract

In the area of the Cenozoic Rhine rift system, crustal and lithospheric thicknesses range between 24 and 35 km, and 60 and 120km, respectively. This rift system transects the deeply truncated Variscan Orogen and superimposed Permo-Carboniferous wrench-induced troughs, and Late Permian and Meso7oic thermal sag basins. At the time of its Westphalian consolidation, the Variscan Orogen was probably characterized by 45-60 km deep-crustal roots that were associated with its Rheno-Hercynian-Saxo-Thuringian, Saxo-Thuringian-Bohemian and Bohemian-Moldanubian sutures, all of which are transected by the Cenozoic Rhine rift system. During the Stephanian-Early Permian wrench-induced disruption of the Variscan Orogen, subducted lithospheric slabs were detached causing upwelling of hot mantle material. During the resulting thermal surge, partial delamination and/or thermal thinning of the continental mantle-lithosphere induced regional uplift. At the same time the Variscan orogenic roots were destroyed and crustal thicknesses reduced to 28-35 km in response to the combined effects of mantle-derived melts interacting with the lower crust, regional erosional unroofing of the crust and, on a more local scale, by its mechanical stretching. Towards the end of the Early Permian, the potential temperature of the asthenosphere returned to ambient levels. With this, regional, long-term thermal subsidence of the lithosphere commenced, controlling the development of a new system of Late Permian and Mesozoic thermal sag basins. However, the evolution of these basins was repeatedly overprinted by minor short-term subsidence accelerations that reflect the build-up of far-field stresses related to rifting in the Tethyan and Atlantic domains. Comparison of observed and modelled subsidence curves suggests that in the area of the Rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the Cretaceous at depths of 100-120km, before it became thermally destabilized again by Cenozoic rifting and plume-related magmatism. Modelled subsidence curves indicate that by the end of Early Permian times the thermal thickness of the remnant mantle-lithosphere ranged between 10 and 50 km in areas that were later incorporated into Mesozoic thermal sag basins; this corresponds to mid-Permian thermal lithosphere thicknesses of 40-80 km.

Evolution of the lithosphere in the area of the Rhine Rift System

“…A northdipping, partly oceanic subduction slab was probably associated with the Bohemian/Moldanubnian suture, whereas the south-dipping Saxo-Thuringian/Bohemian slab had already been detached from the lithosphere during mid-Visean times ( Fig. 4a; Ziegler et al 2004). …”

“…During the Stephanian and Early Permian, dextral translation of Gondwana and Laurussia controlled the Variscan tectonic framework with superimposed ECRIS fault pattern (after Ziegler et al 2004) evolution of the Appalachian-Mauretanides and Scythian orogens and the wrench-induced collapse of the European Variscan Orogen. Transtensional and transpressional wrench deformation of the Variscan Orogen and its northern foreland, as well as associated magmatic activity, abated at the transition to the Late Permian, in tandem with the consolidation of the Appalachian Orogen (Arthaud and Matte 1977;Ziegler 1989Ziegler , 1990Coward 1993;Marx et al 1995;Ziegler and De`zes 2005).…”

“…Although Stephanian-Early Permian wrench deformation locally induced uplift of extensional core complexes (e.g. Massif Central, Montagne Noire; Vanderhaeghe and Teyssier 2001) and subsidence of often narrow, fault-bounded basins, implying high crustal stretching factors, large intervening areas were not significantly extended (Ziegler 1990;van Wees et al 2000;Ziegler et al 2004).…”

“…Strong Stephanian transtensional reactivation of the NE trending German segment of the Rheno-Hercynian/ Saxo-Thuringian suture apparently caused detachment of the subducted continental Rheno-Hercynian lithospheric slab beneath the Mid-German Crystalline Rise; this slab was introduced into the mantle during late Visean to Westphalian times (after mid-Visean detachment of its oceanic part; Ziegler et al 2004). Upwelling of the asthenosphere into the space formerly occupied by this slab, triggered partial melting of the asthenosphere and the remnant mantle-lithosphere, ascent of melts to the base of the crust and anatexis of lower crustal rocks (model of Davies and von Blanckenburg 1995).…”

“…Upwelling of the asthenosphere into the space formerly occupied by this slab, triggered partial melting of the asthenosphere and the remnant mantle-lithosphere, ascent of melts to the base of the crust and anatexis of lower crustal rocks (model of Davies and von Blanckenburg 1995). In conjunction with an ensuing reorganisation of mantle flow patterns, a not-very-active mantle plume apparently welled up to the base of the lithosphere in the area of the eastern parts of the future Southern Permian Basin, causing thermal attenuation of the mantle-lithosphere and magmatic destabilisation of the crust-mantle boundary (van Wees et al 2000;Ziegler et al 2004). A branch of the Southern Permian Basin upwelling system apparently extended from northeast Germany via the Hessian Depression into the area of the Saar-Nahe Trough.…”